generic version cannot include PDF figures with version 1.6 or later

[Wollipolli]

I downloaded the generic version from https://s3.amazonaws.com/dissertate.io/Dissertate-Generic-LaTeX.zip.
My document compiled okay, without throwing any error/warning.
But it would not display some of the included PDF figures.
After a while I realized that those were created with PDF version 1.7, the other ones were 1.5.
Converting them with gs helped. I wonder what the underlying issue is?

gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.5 -o figure_v1.5.pdf figure_v1.7.pdf